In various parts of photoelectronic devices and recording materials, organic polymer materials and organic polymer thin films are used. They are usually produced by using a carbon-based polymer compound having a refractive index of 1.7 or less. In recent years, according to densification of photoelectronic devices or capacity enlargement of recording materials, it is considered that an optical process having a higher numerical aperture (NA) is necessary to be applied. Therefore, also for such organic polymer materials, it is required to have a high refractive index.
As an attempt to make an organic polymer material having a high refractive index, the development of a polymer material containing a polymer compound having, besides a carbon atom, a bromine atom or a sulfur atom is performed. However, by such a technique, a polymer material having a refractive index of over 1.8 has not been yet obtained.
For the purpose of making a polymer material having a further higher refractive index, there is disclosed a resin formed from a high refractive index resin composition in which fine particles of a metal oxide are dispersed in a polymer. For example, there is reported that in a resin formed from a resin composition in which 50% by weight of zirconia (ZrO2) fine particles (having a refractive index of 2.1 in a bulk state) are dispersed in an allyl ether isophthalate resin (having a refractive index of 1.56), it has been calculated that a refractive index of 1.83 can be obtained (see Patent Document 1).
Thus, it is known that a resin in which a metal oxide well known as a substance having a high refractive index is dispersed has a high refractive index. In such a resin, light scattering occurs due to aggregation of metal oxides with each other, so that the resin is likely to be unsuitable for the optical material. For obtaining a homogeneous inorganic fine particles-dispersed resin causing no light scattering capable of solving the above disadvantage, a precise control over the particle diameter of the inorganic fine particle and the organic substituent for surface modification is necessary. In addition, addition of a large quantity of inorganic fine particles to the resin composition for obtaining a resin with high refractive index causes not only a problem of the increased viscosity, but also a fear of impaired transparency (see Patent Document 2).
As a method for solving such a problem of dispersibility of the inorganic fine particle and obtaining a polymer material with high refractive index, there is proposed a method of using a polymer compound in which a metalloid element or metal element with a large atomic number contributing to obtaining a high refractive index is incorporated through a chemical bond.
As an example of such a polymer compound, a polygermane compound having the backbone containing a Ge—Ge bond is reported (Patent Document 3). The polygermane compound is dissolved in a solvent or a resin, so that the polygermane compound can form a homogeneous film causing no light scattering and the film formed therefrom has a high refractive index of approximately 1.8.